In aqueous systems, particularly industrial aqueous systems, corrosion inhibition is necessary for the protection of the metallic parts of the equipment which are exposed to the aqueous solution such as, for example, heat exchangers, pipes, engine jackets, and the like. Corrosion inhibitors are generally added to the aqueous system to prevent metal loss, pitting and tuberculation of such equipment parts.
There are certain disadvantages in using any of the conventional corrosion inhibitors since each present certain drawbacks. For example, chromates are known to be very effective in inhibiting corrosion, but are very toxic. Phosphorus-based corrosion inhibitors such as phosphates and organophosphonates can lead to scale deposition and are also environmentally undesirable. Zinc is not a very effective corrosion inhibitor at low levels (&lt;1 ppm) and is also not very effective at high pH (above 7.5) due to the limited solubility of Zn(OH).sub.2. Molybdates, while known to be effective corrosion inhibitors at high concentrations, are generally not cost-effective. Thus, there exists a need for a non-chromate, non-phosphorus-based, cost-effective corrosion inhibitor for the protection of metal surfaces in contact with aqueous systems.
Rare earth metal cations, which are releasably bound to the surface of a substrate by ion exchange or which are in the form of inorganic salts, have recently been shown to be useful in aqueous systems to inhibit the corrosion of metals. For example, Metals Forum, Vol. 7, No. 7, p. 211 (1984) and U.S. Pat. No. 4,749,550 demonstrated corrosion inhibition using rare earth metal cations of yttrium and the lanthanum series when introduced to the aqueous system in the form of water soluble salts. Effective corrosion inhibition was obtained with a cation concentration as low as 0.4 millimoles per liter (equivalent to 56 ppm), while the preferred lower limit was one millimole per liter (equivalent to 140 ppm). Zh. Prikl. Khim. (Leningrad), 47(10), 2333 (1974) discloses corrosion inhibition with praseodymium and neodymium nitrites.
However, the above referenced inorganic rare earth metal salts have very limited solubilities in aqueous systems, and are, in fact, substantially insoluble in aqueous solutions having pH above 6, or which have high alkalinity or moderate to high hardness. It is an essential requirement for any corrosion inhibitor that it be soluble in the aqueous systems in which the metal is to be protected, not only since solubility permits delivery of the inhibitor to the surface sites where corrosion is occurring but also to avoid deposition of solid particles which can lead to the formation of scale deposits. The foregoing prior art inorganic rare earth metal salts have been found to be ineffective corrosion inhibitors under normal operating conditions of industrial aqueous systems which typically have pHs in the range 7 to 9, which have high alkalinity (as carbonate) and/or which have moderate to high hardness (mineral content) since they are practically insoluble under these conditions.
Other water-insoluble rare earth metals, in the form of carboxylate compounds (U.S. Pat. No. 4,495,225) and rare earth metal-thiourea complexes (Sb. Nauch, Tr. Yaroslav. Gos. Ped. In-t (192)32, have been used in coatings to provide corrosion inhibition. However, coating of the metal surfaces is not always a viable approach to corrosion inhibition particularly where the surface exposed to the corrosive aqueous media is internal to the system, and thus not readily coatable; where the coating of the system would limit or reduce the flow rate of the circulating water after coating; and/or where the coating would detract from the heat transfer efficiency. The above problems present themselves in almost all industrial aqueous applications such- as the internal surfaces of heat exchangers, boilers, cooling towers, pipes and engine jackets. Thus, there is a need for corrosion inhibitors which will work while dissolved in these aqueous systems which inherently have relatively high pHs, high alkalinity and/or moderate to high hardness. Corrosion inhibitors must be soluble, stable and active under the normal operating conditions of these systems. Moreover, these properties must not be adversely affected by the presence of other water treatment compositions or by other conditions which are generally associated with such aqueous systems. These conditions generally include the presence of oxygen in the aqueous system (which accelerates corrosion), a high degree of hardness associated with excessive amounts of calcium, magnesium and carbonate ions, as well as elevated temperature, pH conditions, and the like.